This is a very old folk song which has been re-popularised by Simon & Garfunkel. Though I knew "Parsley, sage, rosemary and thyme" at the time I only really discovered the song watching a DVD of a Sarah Brightman concert.
There are two interpretations of what the writer meant by all those impossible tasks that are set in the verses, one of them is that to be his true love she needs to be willing to attempt the impossible, the other is that she will never be his true love again - making it the oldest break-up song I know. By all means decide for yourself how you wish to see it.
Finally, the full song actually goes for many many more verses but only four of them are found in the Simon & Garfunkel version.
Parsley, sage rosemary and thyme.
Remember me to one who lives there.
She once was a true love of mine.
Tell her to make me a cambric shirt:
Parsley, sage rosemary and thyme;
Without no seams nor needlework,
Then she'll be a true love of mine.
Tell her to find me an acre of land:
Parsley, sage, rosemary and thyme;
Between the salt water and the sea strand,
Then she'll be a true love of mine.
Tell her to reap it with a sickle of leather:
Parsley, sage, rosemary and thyme;
And to gather it all in a bunch of heather,
Then she'll be a true love of mine.
Are you going to Scarborough Fair:
Parsley, sage, rosemary and thyme.
Remember me to one who lives there.
She once was a true love of mine.